Fix more warnings!

GHC 8.4 and later have the `-Wpartial-fields` warning flag, so add it and
fix the warnings.

Turn on and fix more warnings

ihaskell-widgets: Make it compile with ghc 8.4

Fixing these warnings required changes to the file names of the `MessageHeader`
struct which makes this commit an API change. The version in the cabal file
has been bumped accordingly.

-Wall the things

Add `-Wunused-do-bind -Wunused-local-binds -Wunused-top-binds -Wunused-matches`
and fix all the things.

Add `-Wmissing-signatures` to `ghc-options`.

Add `-Wincomplete-patterns` to the `ghc-options` field of the cabal file
and fix all warnings.

GHC 8.4 allows value level operators (in this case list append, '++') to
be used at the type level, so that the singletons package for GHC 8.4
uses this operator whereas previous versions of singletons defined a ':++'
operator for type level append.

Drop support for GHC < 8.0

Support GHC 8.6

update ihaskell labextension version + Haskell-r syntax highlighting